E-cigarettes -- electronic cigarettes -- are tobacco-free devices usually shaped like cigarettes, cigars, or pipes. Users get a hit of vapor containing nicotine or other substances.

They're sold in shopping malls and over the Internet. The FDA says they are unapproved drug delivery devices that cannot legally be sold until proven safe and effective. But last year, the e-cigarette industry filed lawsuits that blocked an FDA ban.

Now the FDA has sent stern warning letters to five companies that market e-cigarettes. The warnings give the firms 15 working days to correct "practices which violate various provisions of the Federal Food, Drug, and Cosmetic Act."

"The companies must demonstrate to the agency that their products are safe and effective for their intended use. Also they must take steps to ensure the quality and purity of the products," said at a news teleconference.

According to the FDA, all five of the firms claim that their products can help people quit smoking cigarettes. Some of the firms also are cited for unsafe manufacturing practices, for "adulterated" products, and for containing ingredients or quantities of ingredients different than those listed on the product label.

One firm, Cixi E-Cig Technology, was cited for products containing drugs in unapproved forms: rimonabant, a weight loss drug not approved in the U.S. or Europe, and a form of tadalafil, an erectile dysfunction drug.

Perhaps more alarmingly, FDA tests found that the firm's Vitamin C E-Liquid product contains dangerous amounts of DEG, a contaminant that the FDA says can cause "serious and irreversible" poisoning.

The companies that received FDA warning letters are:

Cixi E-Cig Technology Inc. Ltd., Las Vegas, Nev.

E-CigaretteDirect LLC, Parker, Colo.

Gamucci America/Smokey Bayou Inc., Jacksonville, Fla.

Johnson Creek Enterprises LLC, Johnson Creek, Wis.

Ruyan America Inc., Minneapolis, Minn.

These are not the only companies marketing e-cigarettes in the U.S. Levy hinted that FDA concerns are not limited to the companies that today received warning letters, but said the FDA is not planning further actions.

"We have not made a decision to remove all e-cigarettes from the market," Levy said. "The action today only concerns the products outlined in the warning letters. We did indicate in the letter to the Electronic Cigarette Association that we are interested in having manufacturers who make these products seek FDA approval of them."

In the warning letter sent to the Electronic Cigarette Association, the FDA noted that e-cigarettes meet the legal definition of both a drug and a device. As such, e-cigarettes would require FDA approval.

To get such approval, companies would have to conduct large-scale clinical trials, submit data showing that use of the products is safe, show that the products are produced according to drug-industry standards, and meet other stringent requirements.